Image formation apparatus and control method thereof

ABSTRACT

An image formation apparatus which includes an image formation unit for performing image formation by developing a latent image formed on an image support body in a printing job with use of a development unit and by transferring the developed image onto a fed recording medium, a collection unit for collecting a residual development agent on the image support body into the development unit, a detection unit for detecting a density of the image formed on the image support body, and a control unit for causing the collection unit to perform the residual development agent collection operation, according to the number of image formation of which image density detected by the detection unit exceeds a predetermined image density is provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image formation apparatus and acontrol method thereof which perform image formation by developing alatent image formed on an image support body with use of a developmentunit and then transferring the developed image onto a fed recordingmedium.

2. Related Background Art

Conventionally, an electrophotographic system has been known in a colorimage formation apparatus. In an image formation process of theelectrophotographic system, initially, a photosensitive drum isuniformly charged by a charging unit, and an electrostatic latent imageis formed by a laser or an LED (light emitting diode). Then the formedelectrostatic latent image is developed by using toner to form a tonerimage, and the formed toner image is transferred onto a recording membersuch as a recording sheet of paper (referred as recording sheethereinafter). Such an operation is performed for each of yellow (Y),magenta (M), cyan (C) and black (K), and the toner images for thesecolors disposed on the sheet are fixed thereto by heat, whereby a colorimage is formed. In this process, after the toner image is transferredonto the sheet, the residual toner on the photosensitive drum iseliminated by a cleaning unit.

It has been demanded in recent years to reduce a manufacturing cost ofthe color image formation apparatus and also to downsize the apparatusitself. For this reason, it has been proposed a so-called cleanerlessapparatus in which any cleaning unit is not provided in the vicinity ofthe image support body such as the photosensitive drum.

In such the cleanerless apparatus, there are provided several methods toeliminate the residual toner on the photosensitive drum. In one method,for example, a contact-type charging unit disposed in the vicinity ofthe photosensitive drum once captures small-quantity residual toner onthe drum after the transferring, changes an electrostatic characteristicof the captured toner, brings back the characteristic-changed toner tothe drum, and then the development unit collects the brought-back tonerand reuses it. According to such the method, it is controlled to collectthe residual toner on the photosensitive drum during a printing job orduring postrotation at the end of the printing job.

During sheet-to-sheet interval in the printing job or during the drumpostrotation at the end of the printing job, the residual toner iscaptured by the charging unit, the captured toner is ejecting, and theejected toner is then collected by the developing unit. However, in suchoperations, to eject the toner from the charging unit (i.e., to bringback the toner once captured by the charging unit to the photosensitivebody) can not overtake. For this reason, the toner is mixed with aferrite carrier acting as a dielectric brush in the charging unit,whereby it is impossible to maintain charging performance of thecharging unit. As a result, there has been a problem that quality of afinally formed image is deteriorated.

In order to solve this problem, it has been thought to increase thefrequency of cleaning operations. However, if the frequency of cleaningoperations is increased, there occurs a problem that to unnecessarilyperform the cleaning operation using the contact-type charging unitdeteriorates the photosensitive drum.

In a case where it is necessary to collect the residual toner on onephotosensitive drum during the printing Job, if the residual toners onthe other drums are collected always at identical timing, an unnecessarycleaning operation is performed to the photosensitive drums for thecolors other than black on condition that continuous printing isperformed in a black/white mode and any image formation is not performedon the drums for the colors other than black. Thus, there occurs aproblem that the photosensitive drum is deteriorated.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an image formationapparatus which can solve the above-described problems.

Another object of the present invention is to provide an image formationapparatus comprising:

an image formation means for performing image formation by developing alatent image formed on an image support body in a printing job with useof a development unit and by transferring the developed image onto a fedrecording medium;

a collection means for collecting a residual development agent on theimage support body into the development unit;

a detection means for detecting a density of the image formed on theimage support body; and

a control means for causing the collection means to perform the residualdevelopment agent collection operation, according to the number of imageformation of which image density detected by the detection means exceedsa predetermined image density.

Still another object of the present invention is to provide an imageformation apparatus comprising:

an image formation means for each color for performing multi-color imageformation by developing a latent image of corresponding color formed oncorresponding one of plural different image support bodies in a printingjob with use of a development unit for corresponding color and bytransferring the developed image onto a fed recording medium;

collection means for each color for collecting a residual developmentagent on the corresponding-color image support body into thecorresponding-color development unit;

plural detection means each for detecting a density of the image formedon the corresponding-color image support body; and

control means for controlling the residual development agent collectionoperation by the collection means to the specific one image formationmeans or to the plural image formation means, according to an additionvalue of the image densities detected by the detection means.

Other objects and features of the present invention will become apparentfrom the following detailed description and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing the structure of an image formationapparatus according to the first embodiment of the present invention;

FIG. 2 is a sectional view for explaining the structure of a chargershown in FIG. 1;

FIG. 3 is a sectional view for explaining the structure of the chargershown in FIG. 1;

FIG. 4 is a block diagram showing the structure of a digital imageprocess unit shown in FIG. 1;

FIG. 5 is a block diagram showing the structure of the digital imageprocess unit shown in FIG. 1;

FIG. 6 is a block diagram showing the structure of a video signal countunit shown in FIG. 5;

FIG. 7 is a flow chart showing a first control method of the imageformation apparatus according to the first embodiment;

FIG. 8 is a flow chart showing a second control method of the imageformation apparatus according to the first embodiment;

FIG. 9 is a flow chart showing a first control method of an imageformation apparatus according to the second embodiment;

FIG. 10 is a flow chart showing a second control method of the imageformation apparatus according to the second embodiment; and

FIG. 11 is a flow chart showing a third control method of the imageformation apparatus according to the second embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, a color image formation apparatus according to the presentinvention will be explained in detail with reference to the attacheddrawings.

First Embodiment

FIG. 1 is a sectional view showing the structure of the image formationapparatus according to the first embodiment of the present invention.The drawing specifically corresponds to a copying machine which consistsof a color reader unit 900 and a color printer unit 1000.

(structure of color reader unit)

In the color reader unit 900, numeral 301 denotes an original mountingglass (referred as platen hereinafter) which is disposed at an upperportion of the unit 900. Numeral 302 denotes an DF (document feeder)which is disposed above the platen 301 and sequentially feeds originaldocuments (merely referred as documents hereinafter) placed on anot-shown original mounting board to the platen 301. It should be notedthat it is possible to dispose a not-shown mirror pressure board insteadof the DF 302.

Numeral 314 denotes a first carriage which contains light sources(halogen lamps) 303 and 304, reflectors 305 and 306 for condensing lightfrom the light sources 303 and 304, and a mirror 307 for reflectinglight reflected or projected from the original.

Numeral 315 denotes a second carriage which contains mirrors 308 and 309for condensing reflection light from the mirror 307 and furtherintroducing the condensed light into a CCD (charge-coupled device) 101.

The CCD 101 converts the reflection light input through the mirrors 307,308 and 309 and a lens 310 into an electrical signal. In a color sensor,a one-line CCD in which R (red), G (green) and B (blue) filters aredisposed in line in that order, a three-line CCD in which R, G and Bfilters are disposed in respective lines, a CCD in which filters aredisposed on one chip, or a CCD which has independent filters may be usedas the CCD 101.

Numeral 311 denotes a base on which the CCD 101 is installed. Numeral312 denotes a digital image process unit (merely referred as imageprocess unit hereinafter) which includes later-described componentsexcept for the CCD 101 shown in FIG. 4, a later-described binaryconversion unit 501 shown in FIG. 5, later-described video signal countunits 520 to 523 shown in FIG. 5, later-described delay units 502 to 505shown in FIG. 5, a not-shown CPU (central processing unit), a not-shownROM (read-only memory), a not-shown RAM (random access memory), and thelike. The CPU of the image process unit 312 entirely controls thecopying machine on the basis of a program stored in the ROM. Numeral 313denotes an I/F (interface) unit which acts as an interface for anotherIPU (image processing unit) or the like. Image information which wasinput from an external apparatus such as a host computer or the likethrough a predetermined communication means is transferred to the imageprocess unit 312 by the I/F unit 313, whereby image formation can beperformed by a color printer unit on the basis of the transferredinformation.

The first carriage 314 is mechanically moved by a drive unit 316 in adirection perpendicular to an electrical scanning direction (i.e., mainscanning direction) of the CCD 101 at speed V. Also, the second carriage315 is mechanically moved by the drive unit 316 in the same direction atspeed V2. Thus, the face of the original is entirely scanned (i.e., insub scanning direction).

(structure of color printer unit)

In the color printer unit 1000, numerals 317, 318, 319 and 320 denote anY image formation unit, an M image formation unit, a C image formationunit and K image formation unit, respectively. The Y image formationunit 317 contains a photosensitive drum 342, a charger 321, an LED unit(or LED array) 210, a development unit 322, an auxiliary charger 360 anda transfer drum (or transfer charger) 323. The M image formation unit318 contains a photosensitive drum 343, a charger 324, an LED unit 211,a development unit 325, an auxiliary charger 361 and a transfer drum (ortransfer charger) 326. The C image formation unit 319 contains aphotosensitive drum 344, a charger 327, an LED unit 212, a developmentunit 328, an auxiliary charger 362 and a transfer drum (or transfercharger) 329. The K image formation unit 320 contains a photosensitivedrum 345, a charger 330, an LED unit 213, a development unit 331, anauxiliary charger 363 and a transfer drum (or transfer charger) 332.

The chargers 321, 324, 327 and 330 contain rotatively movable chargingsleeves 370, 371, 372 and 373, respectively. Each of the sleeves 370,371, 372 and 373 includes a not-shown magnetic field generation unitwhich generates a magnetic field by applying an AC bias voltage.

The development units 322, 325, 328 and 331 are provided withdevelopment sleeves 355, 356, 357 and 358, respectively. Each of thesleeves 355, 356, 357 and 358 includes a not-shown magnetic fieldgeneration unit which generates a magnetic field by applying an AC biasvoltage.

Since the structures of the image formation units 317, 318, 319 and 320are identical, only the Y image formation unit 317 will be explained indetail, and explanation of the other units will be omitted.

The auxiliary charger 360 and the charger 321 uniformly charges thesurface of the photosensitive drum 342 to prepare latent imageformation. The LED unit 210 forms the latent image on the drum 342 byusing light. The development unit 322 develops the latent image on thesurface of the drum 342 to form a toner image.

The transfer charger 323 which is disposed below the development unit322 to pinch a transfer belt 333 discharges from the back side of thebelt 333 to transfer the toner image on the photosensitive drum 342 to arecording sheet or the like on the belt 333.

Numeral 338 and 339 denotes pickup rollers which fed one by one transfermembers such as the transfer sheets held in cassettes 340 and 341 ontothe moving transfer belt 333 through sheet feed rollers 336 and 337,respectively. The feed rollers 336 and 337 once stop the transfermembers fed by the pickup rollers 338 and 339 respectively, and thensupply them onto the belt 333 at predetermined timing. Numeral 348denotes a transfer belt roller which drives the transfer belt 333disposed below the image formation units 317, 318, 319 and 320.

Numeral 346 denotes a charger which charges the recording sheet or thelike to be fed to the transfer belt 333. Numeral 347 denotes a sheetleading edge sensor which detects the leading edge of the recordingsheet fed to the transfer belt 333. A detection signal from the sensor347 is transferred from the color printer unit 1000 to the color readerunit 900 to be used as a sub-scanning sync signal when a video signal istransferred from the unit 900 to the unit 1000.

Numeral 349 denotes a charge elimination charger which eliminateselectric charge on the recording sheet or the like passed the K imageformation unit 320. Numeral 350 denotes a separation charger which isdisposed adjacent to the charge elimination charger 349 to prevent imagederangement due to separation discharging occurred when the recordingsheet or the like is separated from the transfer belt 333.

Numerals 351 and 352 denote prefixing chargers which charge therecording sheet or the like. Numeral 334 denotes a fixing unit whichthermally fixing the toner image on the recording sheet after the sheetis charged by the chargers 351 and 352, and then discharges the sheet toa sheet discharge tray 335. Numerals 353 denote internal and externalcharge eliminators which eliminates electric charge on the transfer belt333.

Hereinafter, operations of the respective units in the color printerunit 1000 will be explained.

Initially, the photosensitive drum 342 is charged by the auxiliarycharger 360 and the charger 321. The surface of the drum 342 isuniformly charged by the charger 321 to prepare the latent imageformation.

Next, the latent image is formed on the photosensitive drum 342 by thelight from the LED array 210, and the formed latent image is developedby the development unit 322 to form the toner image.

The development unit 322 includes the development sleeve 355 whichperforms the development by applying a development bias voltage. Thetransfer charger 323 which is disposed below the development unit 322 topinch the transfer belt 333 discharges from the back side of the belt333 to transfer the toner image on the photosensitive drum 342 to therecording sheet or the like on the belt 333.

After the transfer is performed, developer (i.e., toner) residual on thephotosensitive drum 342 is once captured by the charger 321 acting as afirst collection means, an electrostatic characteristic of the captureddeveloper is changed, and then the developer is again brought back tothe drum 342. Then the developer is collected and reused by thedevelopment unit 322 acting as a second collection means.

Next, the procedure to form the image on the recording sheet or the likewill be explained. The recording sheet or the like held in the cassette340 or 341 is fed one by one with use of the pickup roller 338 or 339,and then the sheet is supplied onto the moving transfer belt 333 by thesheet feed roller 336 or 337. The transfer belt 333 which is disposedbelow the Y, M, C and K image formation units 317, 318, 319 and 320 isdriven by the transfer belt roller 348.

The leading edge of the recording sheet fed to the transfer belt 333 isdetected by the sheet leading edge sensor 347. The detection signal fromthe sensor 347 is transferred from the color printer unit to the colorreader unit to be used as the sub-scanning sync signal when the videosignal is transferred from the color reader unit to the color printerunit.

After then, the recording sheet is carried by the transfer belt 333, andthus Y, M, C and K toner images are sequentially formed on the sheet inthat order by the image formation units 317, 318, 319 and 320.

The recording sheet passed the K image formation unit 320 is chargeeliminated by the charge elimination charger 349 such that the sheet canbe easily separated from the belt 333. Then the sheet is actuallyseparated from the belt 333. The separation charger 350 is disposedadjacent to the charge elimination charger 349 to prevent imagederangement due to separation discharging occurred when the recordingsheet is separated from the transfer belt 333.

In order to compensate for adsorption of the toner and prevent the imagederangement, the separated recording sheet is charged by the prefixingchargers 351 and 352, the toner image is thermally fixed by the fixingunit 334, and then the sheet with the fixed image is discharged onto thesheet discharge tray 335. Also, the transfer belt is charge eliminatedby the internal and external charge eliminators 353.

FIG. 2 is a sectional view for explaining the structure of the chargershown in FIG. 1. In FIG. 2, the parts same as those in FIG. 1 arerespectively added with the same numerals.

As shown in FIG. 2, by rotating the charging sleeve 370 along thedirection reverse to the rotational direction of the photosensitive drum342, the charger 321 forms a dielectric brush by a non-resistive ferritecarrier 502 to uniformly charge the surface of the drum 342 with chargedparticles, thereby preparing the latent image formation. Further, thecharger 321 once captures toner 503 residual on the drum 342 after theimage transferring is performed, changes the electrostaticcharacteristic of the toner, and again brings back it to the drum 342.

FIG. 3 is a sectional view for explaining the structure of the chargershown in FIG. 1. In FIG. 3, the parts same as those in FIG. 1 arerespectively added with the same numerals.

In FIG. 3, alphabetical symbol a denotes a charging position at whichthe charger 321 uniformly charges the surface of the photosensitive drum342 to prepare the latent image formation. Alphabetical symbol b denotesan exposure position at which the LED 210 exposes the surface of thedrum 342 to form the electrostatic latent image. Alphabetical symbol cdenotes a development position at which the development sleeve 355 ofthe development unit 322 develops the electrostatic latent image on thesurface of the drum 342 with use of the developer to form the developerimage. Alphabetical symbol d denotes a transfer position at which thetransfer charger 323 transfers the developer image on the surface of thedrum 342 to the recording sheet or the like.

As shown in FIG. 3, the charger 321 uniformly charges the surface of thephotosensitive drum 342 at the charging position a to prepare the latentimage formation. Then the LED 210 exposes the surface of the drum 342 atthe exposure position b to form the electrostatic latent image. Then thedevelopment sleeve 355 of the development unit 322 develops theelectrostatic latent image on the surface of the drum 342 with use ofthe developer at the development position c to form the developer image.Then the transfer charger 323 transfers the developer image on thesurface of the drum 342 to the recording sheet or the like at thetransfer position d.

FIGS. 4 and 5 are block diagrams showing the structure of the imageprocess unit 312 shown in FIG. 1.

In the drawings, numeral 402 denotes a clamp and amplifier and SH(sample and hold) and A/D (analog-to-digital conversion) unit. The unit402 performs a sample and hold process to electrical signals (i.e.,analog image signal) converted from the reflection light of the originalby the CCD 101, clamps a dark level of the analog image signal toreference potential, amplifies the potential by a predeterminedquantity. It should be noted that the order of such processes is notlimited to this. Namely, these processes may be performed in anotherorder. Then the unit 402 performs AID conversion to convert the obtainedsignals into eight-bit R, G and B digital signals.

Numeral 403 denotes a shading unit which performs shading correction andblack correction to the R, G and B signals input from the clamp andamplifier and SH and AD unit 402. Numeral 404 denotes a binding and MTF(modulation transfer function) correction and original detection unit.If the CCD 101 is the three-line CCD, image reading positions of thethree lines are different from others. Therefore, the unit 404 performsa binding process to adjust a delay quantity for each line in accordancewith reading speed, corrects signal timing such that the image readingpositions of the three lines become identical. Since an MTF for thereading changes according to the reading speed and a magnificationchange rate, the unit 404 performs MTF correction to correct such achange. Then the unit 404 recognizes the size of the original on theplaten.

Numeral 405 denotes an input masking unit which corrects a spectralcharacteristic of the CCD 101 and spectral characteristics of the lightsources 303 and 304 and the reflectors 305 and 306, on the basis of thedigital signals of which reading position timing have been corrected bythe binding and MTF correction and original detection unit 404. Theoutputs of the input masking unit 405 are input to a selector 406. Theselector 406 can change the inputs between the signals from the unit 405and external I/F signals from an external I/F unit 414.

Numeral 415 denotes a background elimination unit which performsbackground elimination to signals output from the selector 406.

Numeral 416 denotes a black character judgment unit which judges whetheror not a target character in the original is a black character, andgenerates a black character signal on the basis of the original.

Numeral 407 denotes a color gamut mapping and background elimination andlogarithmic conversion unit which is composed of a color gamut mapping(or color space compression) unit, a background elimination unit and alogarithmic conversion unit. The color gamut mapping unit judges whetheror not the read image signal is within a gamut capable of beingreproduced by the printer. If the image is within the gamut, the gamutmapping unit retains the signal as it is. On the other hand, if thesignal is not within the gamut, the unit corrects the signal to bewithin the gamut.

Then the background elimination unit performs a background eliminationprocess, and the logarithmic conversion unit performs a logarithmicconversion to convert the R, G and B signals into C, M and Y signals.

Numeral 408 denotes a delay unit which adjusts timing of the outputsignals of the color gamut mapping and background elimination andlogarithmic conversion unit 407 so as to match the timing of thesesignals and timing of the signal generated by the black characterjudgment unit 416. Numeral 409 denotes a moire elimination unit whicheliminates moire of the above two kinds of signals (i.e., signalgenerated by unit 416 and output signal of unit 407). Numeral 410denotes a magnification change process unit which performs amagnification change process in the main scanning direction.

Numeral 411 denotes an UCR (under color removal) and masking and blackcharacter reflection unit which is composed of an UCR unit, a maskingunit and a black character reflection unit. The UCR unit performs an UCRprocess to the C, M and Y signals processed by the magnification changeprocess unit 410 to generate the C, M, Y and K signals. Then the maskingunit corrects these signals to be matched with the printer outputting,and the black character reflection unit feeds back the judgment signalgenerated by the black character judgment unit 416 to the C, M, Y and Ksignals.

Numeral 412 denotes a gamma correction unit which performs densityadjustment to the signals processed by the UCR, and masking and blackcharacter reflection unit 411. Numeral 413 denotes a filter unit whichperforms a smoothing process or an edge process to the signals outputfrom the gamma correction unit 412 and then outputs the processedsignals to the binary conversion unit 501.

The eight-bit multivalue signals processed as above are converted intobinary signals by the later-described binary conversion unit 501 (FIG.5). As a conversion method in the unit 501, any of a dither method, anerror diffusion method, and an improved error diffusion method can beused.

Next, in FIG. 5, the binary conversion unit 501 binarizes the signalsfrom the filter unit 413. Numerals 520, 521, 522 and 523 denote thevideo signal count units which can count the number of light emissionelements in the LED for each color image based on the signals input fromthe binary conversion unit 501.

Numerals 502, 503, 504 and 505 denote the delay units which delay thebinarized image signals in accordance with distances between the sheetleading edge sensor 347 and the respective image formation positions.Numerals 506, 507, 508 and 509 denote the LED drive units which generatethe signals to drive the LED units 210, 211, 212 and 213 respectively.

Hereinafter, an operation of each unit will be explained.

The light from the light sources 303 and 304 is reflected on theoriginal put on the platen 301, introduced into the CCD 101, andconverted into the electrical signal.

The converted electrical signal (i.e., analog image signal) is input tothe image process unit 312. In the unit 312, by the clamp and amplifierand SH and A/D unit 402, the input signal is subjected to the sample andhold process, the dark level of the analog image signal is clamped tothe reference potential, and the potential is amplified by thepredetermined quantity. It should be noted that the order of suchprocesses is not limited to this. Then the signal is subjected to theA/D conversion, thereby obtaining the eight-bit R, G and B digitalsignals.

The R, G and B signals are subjected to the shading correction and theblack correction by the shading unit 403, and then subjected to thebinding process by the binding and MTF correction and original detectionunit 404. If the CCD 101 is the three-line CCD, the image readingpositions of the three lines are different from others. Therefore, inthe binding process, the delay quantity for each line is adjustedaccording to the reading speed, and the signal timing is corrected suchthat the image reading positions of the three lines become identical.Since the MTF for the reading changes according to the reading speed andthe magnification change rate, the MTF correction is performed tocorrect such the change. Then the size of the original on the platen isrecognized.

The digital signals of which reading position timing have been correctedare input to the input masking unit 405. In the unit 405, the spectralcharacteristic of the CCD 101 and the spectral characteristics of thelight sources 303 and 304 and the reflectors 305 and 306 are corrected.The outputs of the input masking unit 405 are input to the selector 406which can change the inputs between the signals from the unit 405 andthe external I/F signals.

The signals output from the selector 406 are input to the color gamutmapping and background elimination and logarithmic conversion unit 407and to the background elimination unit 415. The signals input to theunit 415 are subjected to the background elimination, and then input tothe black character judgment unit which judges whether or not the targetcharacter in the original is the black character and generates the blackcharacter signal based on the original. On the other hand, it is judgedbased on the signals input to the unit 407 whether or not the read imagesignal is within the gamut capable of being reproduced by the printer.If the image is within the gamut, the unit 407 retains the signal as itis, while if the signal is not within the gamut, the unit 407 correctsthe signal to be within the gamut. Then the background eliminationprocess is performed, and the logarithmic conversion to convert the R, Gand B signals into the C, M and Y signals is performed.

The timing of the signals output from the color gamut mapping andbackground elimination and logarithmic conversion unit 407 is adjustedsuch that it matches with the timing of the signal generated by theblack character judgment unit 416. These two kinds of signals aresubjected to the moire elimination by the moire elimination unit 409,and then subjected to the magnification change process in the mainscanning direction by the magnification change process unit 410.

Then, in the UCR and masking and black character reflection unit 411,the C, M and Y signals processed by the magnification change processunit 410 are further subjected to the UCR process to generated the C, M,Y and K signals, and these signals are subjected to the masking processto be matched with the printer outputting. Further, the signal generatedby the black character reflection unit 416 is fed back to the C, M, Yand K signals.

The signals processed by the unit 411 are then subjected to the densityadjustment by the gamma correction unit 412, and then subjected to thesmoothing process or the edge process by the filter unit 413.

Then the signals from the filter unit 413 are binarized by the binaryconversion unit 501, and transferred to the video signal count units520, 521, 522 and 523. In each of the units 520, 521, 522 and 523, thetotal number of light emission elements in the LED can be counted foreach color image.

After then, the binarized image signals are delayed by the delay units502, 503, 504 and 505 in accordance with the distances between the sheetleading edge sensor 347 and the respective image formation positions,and the delayed signals are transferred to the LED drive units 506, 507,508 and 509 which generate the signals respectively to drive the LEDunits 210, 211, 212 and 213.

Next, a method to interrupt the printing job and eliminate the residualtoner on the photosensitive drum (i.e., collect the toner and bring itback to the development unit) according to the image density will beexplained in detail.

1. Method to detect image density

As the image density, the value which is obtained by dividing an area ofthe recording medium into the total number of light emission elements inthe LED counted for each color image by the video signal count units 520to 523 in FIG. 5 is used.

FIG. 6 is a block diagram showing the structure of the video signalcount unit 520 shown in FIG. 5. It should be noted that the structuresof the video signal count units 521 to 523 are identical with that ofthe unit 520.

In FIG. 6, numeral 700 denotes an image signal which is transferred fromthe binary conversion unit 501. Numerals 701 to 708 denote 29-bitcounters which count in parallel eight-bit image signals of one imageobtained from the signal 700. Numeral 709 denotes a 32-bit adder whichadds the counted results of the counters 701 to 708 to obtain the totalnumber of light emission elements in the LED as 32-bit data.

Such a process is performed for each image formation to obtain the totalnumber of light emission elements in the LED, the obtained number isdivided by the area of the recording medium at that time, and the valueobtained by such division is stored in a not-shown RAM of the imageprocess unit 312 as the image density. Further, the number of images ofwhich densities exceed a threshold value is counted, and also the imagedensities are added up. Then, when the counted number reaches apredetermined number, a flag is set in the not-shown RAM of the unit312. At a time of registration ON (i.e., at timing of supplying therecording member onto the transfer belt 333 by the sheet feed rollers336 and 337), if the flag stands, the printing job is temporarilyinterrupted until the flag is reset, and the residual toner elimination(i.e., collection) process described below is performed.

2. Method to eliminate (or collect) residual toner on photosensitivedrum in temporary interruption of print job

Hereinafter, the residual toner elimination (collection) process whichis performed on the photosensitive drum when the flag representing thatthe number of images of which image densities exceed the threshold valuereaches the predetermined number is set in the not-shown RAM in theimage process unit 312 will be explained.

In order to clean off the residual toners on the photosensitive drums342, 343, 344 and 345, the drums 342, 343, 344 and 345 are driven, e.g.,a DC bias voltage of “−700V” and an AC bias voltage of “1.1 KVpp”, “1kHz” and “50%” duty rectangle wave are applied to the chargers 321, 324,327 and 330 to drive the charging sleeves 370, 371, 372 and 373, and,e.g., a DC bias voltage of “−550V” and an AC bias voltage of “1 KVpp”,“2.2 kHz” and “50%” duty rectangle wave are applied to the developmentunits 322, 325, 328 and 331 to drive the charging sleeves 355, 356, 357and 358, respectively.

By doing so, the chargers 321, 324, 327 and 330 once capture the tonerson the photosensitive drums 342, 343, 344 and 345, change theirelectrostatic characteristics, and bring back them onto the drums 342,343, 344 and 345, respectively. Then the development units 322, 325, 328and 331 collect the respective toners.

Such a toner collection operation to be performed during the printingjob is interrupted copes with a case where the toner captured by thecharger is not sufficiently ejected in the high-density image formation.In addition, the charging sleeves 370, 371, 372 and 373 are rotativelydriven in the state that only the AC bias voltage of the chargers 321,324, 327 and 330 is OFF (i.e., in the state that magnetic fieldgeneration units in the sleeves are not driven) to eject the tonerscaptured in the chargers. Then the development units 322, 325, 328 and331 collect the respective toners.

As above, the AC and DC voltages are used as the bias voltages to beapplied to the charger 321 (324, 327, 330). When applying the DCvoltage, the toner in the charger is ejected to the photosensitive drum342 (343, 344, 345), while when applying the AC voltage, the residualtoner on the drum 342 (343, 344, 345) is attracted to the charger 321(324, 327, 330).

Therefore, in case of mainly ejecting the toner remained in the charger321 (324, 327, 330), only the AC voltage is OFF.

In the image formation apparatus according to the present embodiment,for example, if the image formation of which image density is 6% isperformed, it is necessary to interrupt the printing job once per 1000sheets and eject or discharge the residual toner in the charger.

Therefore, the image density values are added up, and the printing jobis interrupted when the obtained value exceeds 6000 (6%×1000 sheets).However, if the image formation of which image density is 2% or less,since the latent image is completely transferred to a recording agentand thus the toner is hardly retained, the image density values are notadded up.

If the residual toner elimination (collection) process ends, the flag(representing that the number of images of which image densities exceedthe threshold value reaches the predetermined number) which has been setin the not-shown RAM of the image process unit 312 is reset.

After then, the printing job once interrupted restarts, whereby therecording member is supplied to the transfer belt 333 by the sheet feedrollers 336 and 337.

Hereinafter, a method to control the image formation apparatus accordingto the present invention will be explained with reference to FIGS. 7 and8.

FIG. 7 is a flow chart showing a first control method of the imageformation apparatus according to the present invention. The firstcontrol method corresponds to the image density detection process and isperformed and controlled by a not-shown CPU of the image process unit312 shown in FIG. 1 on the basis of a program stored in a ROM or anotherrecording medium. In FIG. 7, numerals (1) to (7) represent respectivesteps.

Initially, if data representing the total number of light emissionelements of the LED (referred as LED light emission element total numberdata hereinafter) is output from the 32-bit adder in each imageformation, the LED light emission element total number data is dividedby the area of the recording member at that time to calculate the imagedensity (1).

Next, it is judged whether or not the image density exceeds thethreshold value (2). If judged that the density does not exceed thethreshold value, the process end, while if judged that the densityexceeds the threshold value, the counter stored in the not-shown RAM ofthe image process unit 312 is counted up (3).

Next, it is judged whether or not the counted value reaches thepredetermined number (4). If judged that the counted value reaches thepredetermined number, the flow advances to a step (7).

On the other hand, if judged in the step (4) that the counted value doesnot reach the predetermined number, the addition value (i.e., the sum)of the image density is stored in the not-shown RAM of the image processunit 312 (5). Then it is judged whether or not the addition valuereaches the predetermined threshold value (6). If judged that theaddition value does not reach the predetermined threshold value, theprocess ends, while if the addition value reaches the predeterminedthreshold value, then the flag is set in the not-shown RAM of the imageprocess unit 312 and also the addition value is cleared (7), and theprocess ends.

FIG. 8 is a flow chart showing a second control method of the imageformation apparatus according to the present invention. The secondcontrol method corresponds to the residual toner elimination(collection) process on the photosensitive drum in the temporaryinterruption of the printing job and is performed and controlled by thenot-shown CPU of the image process unit 312 shown in FIG. 1 on the basisof a program stored in the ROM or another recording medium. In FIG. 8,numerals (11) to (15) represent respective steps.

Initially, at the time of registration ON (i.e., at timing of supplyingthe recording member to the transfer belt 333 by the sheet feed rollers336 and 337), it is judged whether or not the flag representing that thenumber of images of which image density exceeds the threshold valuereaches the predetermined number is set (11). If judged that the flag isnot set, the printing job is maintained as it is (i.e., starting tosupply the recording member onto the transfer belt 333 by the sheet feedrollers 336 and 337).

On the other hand, if judged in the step (11) that such the flag is set,the printing job is temporarily interrupted (12), and the cleaningoperation is performed (13).

If the cleaning operation ends, the flag (representing that the numberof images of which image densities exceed the threshold value reachesthe predetermined number) which has been set in the not-shown RAM of theimage process unit 312 is reset (14). Then the printing job onceinterrupted in the step (12) restarts, whereby the recording member issupplied to the transfer belt 333 by the sheet feed rollers 336 and 337(15).

By the above operation, in the image formation apparatus which does nothave a cleaning-dedicated device, the residual toner elimination(collection) process on the photosensitive drum is performed at thetiming determined according to the image density to eject the tonerremaining in the charging unit. Thus, it is possible to prevent that thetoner is mixed with the ferrite carrier acting as the dielectric brushin the charging unit, thereby maintaining the charging performance,preventing image deterioration, and providing a high-quality image.Also, since the toner elimination (collection) operation is performed atthe timing according to the image density, it is possible to preventdeterioration of the photosensitive drum.

Further, since a development agent collected into each of the respectivecolor development units during the residual toner elimination(collection) operation can be reused, it is possible to prevent that thedevelopment agent is used wastefully, thereby satisfactorily saving thedevelopment agents.

Modification of First Embodiment

In the above-described first embodiment, it is explained the case wherethe printing job is temporarily interrupted to perform the tonerelimination (collection) process by judging the number of images ofwhich densities exceed the predetermined image density and the additionvalue (the sum) of the image densities. However, it may be structuredthat a time necessary for the toner elimination (collection) process onthe drum is changed according to the addition value of the imagedensities.

By doing so, it is possible to improve toner elimination (collection)process efficiency.

Further, in the first embodiment, it is explained the case where theimage density is obtained by using the total number of light emission(i.e., video count) by the LED light emission elements of the imageformation apparatus which causes the LED light emission elements to emitthe light to form the latent image on the photosensitive member.However, in an apparatus which forms the latent image on thephotosensitive body by scanning a laser beam, it may be structured thatthe image density is obtained by using the total number of laserlighting signals (i.e., video count).

Further, it may be structured that a potential sensor 550 is provided inthe vicinity of the photosensitive drum to measure the potential on thedrum, thereby obtaining the image density.

Further, in the first embodiment, it is explained the case where eachcolor image is formed onto the corresponding one of the pluralphotosensitive drums, and the images of respective colors aresuperimposed to form the multi-color image. However, it may bestructured that a monochrome image is formed onto one photosensitivedrum and then the respective color images sequentially formed on thatdrum are face-sequentially superimposed to form the multi-color image.

Thus, in a color image formation apparatus wherein a dedicated cleaningdevice for an image support body is not disposed in the vicinity of thatbody, the density of the image to be formed is detected, a printing jobis temporarily interrupted according to the number of image formation ofwhich image densities exceed a predetermined value, and thus a residualtoner elimination (collection) operation is forcedly performed. By doingso, in a case where the number of printing of which density exceeds apredetermined image density exceeds a predetermined number, it ispossible to temporarily interrupt the printing job and perform theresidual toner elimination (collection).

As above, in the color image formation apparatus which does not use thecleaning-dedicated device, by eliminating the residual toner on thephotosensitive drum with the elimination (collection) operationaccording to the image density, it is possible to prevent that toner ismixed with a ferrite carrier acting as a dielectric brush in a chargingunit, thereby maintaining charging performance. Further, since anunnecessary elimination (collection) operation is not performed, it ispossible to prevent deterioration of the photosensitive drum.

Further, since a development agent collected into each of the respectivecolor development units during the residual toner elimination(collection) operation can be reused, it is possible to prevent that thedevelopment agent is used wastefully, thereby satisfactorily saving thedevelopment agents.

According to the above-described first embodiment, the detection meansdetects the density of the image formed on the image support body by theprinting job, and the control means interrupts the printing job andcontrols the residual development agent collection operation by thecollection means in accordance with the number of image formation ofwhich image density detected by the detection means exceeds thepredetermined image density. Therefore, the unnecessary residualdevelopment agent collection operation is restricted, and the residualdevelopment agent collection operation is performed at the timingaccording to the image density, whereby it is possible to preventdeterioration of the image support body.

Further, the collection means includes the first collection means(charger (magnetic field generation means, sleeve, low-resistancecarrier)) which once captures the residual development agent on theimage support body, changes its electrostatic characteristic of thecaptured toner, and brings back the characteristic-changed developmentagent to the image support body, and the second collection means(development unit) which collects the brought-back development agentinto the development unit of each color. Therefore, it is possible toprevent that the development agent is mixed with the ferrite carrieracting as the dielectric brush in the charger, thereby maintainingcharging performance.

Further, in the residual development agent collection operation to beperformed during interruption of the printing job, the control meanscontrols the charger such that, after the charger ejects theonce-captured development agent to the image support body, the chargerfurther ejects the development agent by driving the sleeve in the statethat the magnetic field generation means is not driven. Therefore, evenin the residual development agent collection operation duringinterruption of the printing job, it is possible to sufficiently ejectthe development agent which was used in the high-density image formationand captured into the charger in the development agent collectionoperation. Thus, it is possible to prevent that the development agent ismixed with the ferrite carrier acting as the dielectric brush in thecharger, thereby maintaining charging performance.

Further, the control means calculates the addition value of the imagedensities detected by the detection means, and then controls the timenecessary for the residual development agent collection by thecollection means in accordance with the addition value of the imagedensities calculated. Therefore, it is possible to improve efficiency inthe development agent collection process.

Further, in the control method for the image formation apparatus whichcomprises the respective-color image formation means for developing thelatent images formed on the different plural image support bodies forrespective color components in the printing job with use of respectivecolor development units and performing the multi-color image formationby transferring the developed latent images on the fed recording mediaand the respective-color collection means for collecting the residualdevelopment agents on the image support bodies into the respectivedevelopment units, there are provided the detection step of detectingthe image density to be formed on the image support body, and thecollection step of interrupting the printing job and performing theresidual development agent collection operation by the collection meansin accordance with the number of image formation of which imagedensities detected in the detection step exceed the predetermined imagedensity. Therefore, the unnecessary residual development agentcollection operation is restricted, and the residual development agentcollection operation is performed at the timing according to the imagedensity, whereby it is possible to prevent deterioration of the imagesupport body.

Therefore, it is possible to restrict the unnecessary residualdevelopment agent collection operation and prevent deterioration of theimage support body, and also it is possible to maintain chargingperformance of the charger and form a high-quality image.

Second Embodiment

Subsequently, it will be explained in detail a case where a printing jobis interrupted according to density of an image formed in any imagesupport body, and it is determined based on the image support bodycaused such interruption whether a residual toner collection operationfor the specific image support body is to be performed or a residualtoner collection operation for the plural image support bodies is to beperformed.

Hereinafter, the method to interrupt the printing job and determinebased on the image support body caused the interruption whether theresidual toner collection operation for the specific image support bodyis to be performed or the residual toner collection operation for theplural image support bodies is to be performed will be explained withreference to flow charts shown in FIGS. 9, 10 and 11. It should be notedthat a hardware structure in the second embodiment is the same as thatin the first embodiment shown in FIGS. 1 to 6.

Further, in the second embodiment, two means for interrupting theprinting job are provided. One is to interrupt the printing job when animage density addition value for K (black) exceeds a predeterminedvalue, and the other is to interrupt the printing job when any one ofimage density addition values for Y (yellow), M (magenta) and C (cyan)exceeds a predetermined value.

FIG. 9 is the flow chart showing a first control method of the imageformation apparatus according to the second embodiment. The firstcontrol method corresponds to an image density detection process in a Kimage formation and is performed and controlled by a not-shown CPU ofthe image process unit 312 shown in FIG. 1 on the basis of a programstored in a ROM or another recording medium. In FIG. 9, numerals (21) to(23) represent respective steps.

Initially, in the K image formation, if data representing the totalnumber of light emission elements of the LED (referred as LED lightemission element total number data hereinafter) is output from the32-bit adder 709 in the video signal count unit 523, the LED lightemission element total number data is divided by the area of a recordingmember at that time to calculate a K image density, and a addition valueDK of the K image density is stored in a not-shown RAM of the imageprocess unit 312 (21). Then it is judged whether or not the additionvalue of the K image density reaches a predetermined threshold value,e.g., 6000 (22). If judged that the value does not reach the thresholdvalue, the flow returns to the step (21), while if judged that the valuereaches the threshold value, a K interruption flag is set in thenot-shown RAM of the image process unit 312 and also the K additionvalue DK is cleared (23), and then the process ends.

FIG. 10 is the flow chart showing a second control method of the imageformation apparatus according to the second embodiment. The secondcontrol method corresponds to an image density detection process in Y, Mand C image formation and is performed and controlled by the not-shownCPU of the image process unit 312 shown in FIG. 1 on the basis of aprogram stored in the ROM or another recording medium. In FIG. 10,numerals (31) to (33) represent respective steps.

Initially, in any of the Y, M and C image formation, if LED lightemission element total number data is output from the 32-bit adder 709in a video count unit corresponding to any of the video signal countunits 520, 521 and 522, the LED light emission element total number datais divided by the area of a recording member at that time to calculatean image density of corresponding color, and a addition value (DY, DM,DC) of the corresponding color is stored in the not-shown RAM of theimage process unit 312 (31). Then it is judged whether or not theaddition value (DY, DM, DC) of the corresponding color reaches apredetermined threshold value, e.g., 6000 (32). If judged that the valuedoes not reach the threshold value, the flow returns to the step (31),while if judged that the value reaches the threshold value, an YMCAinterruption flag is set in the not-shown RAM of the image process unit312 and also the Y addition value DY, the M addition value DM and the Caddition value DC are cleared (33), and then the process ends.

FIG. 11 is the flow chart showing a third control method of the imageformation apparatus according to the second embodiment. The thirdcontrol method corresponds to the residual tone collection operationcontrol process at a time when a recording member is supplied, and isperformed and controlled by the not-shown CPU of the image process unit312 shown in FIG. 1 on the basis of a program stored in the ROM oranother recording medium. In FIG. 11, numerals (41) to (44) representrespective steps.

Initially, in the case where the recording member is fed onto thetransfer belt 333 by the sheet feed rollers 336 and 337, the state ofthe K interruption flag or the YMC interruption flag is obtained fromthe not-shown RAM of the image process unit 312 (41).

Then it is judged whether or not the obtained K interruption flag or theYMC interruption flag is in a reset state (42). If judged that the flagis in the reset state, the printing job is maintained as it is (i.e.,starting to supply the recording member onto the transfer belt 333 bythe sheet feed rollers 336 and 337) (44).

On the other hand, if judged in the step (42) that the flag is not inthe reset state (i.e., in a set state), the printing job is temporarilyinterrupted, and the residual toner collection operation to thecorresponding photosensitive drum is performed. Then, when the operationends, the corresponding flag is reset, and the once-interrupted printingjob is restarted (i.e., restarting to supply the recording member ontothe transfer belt 333 by the sheet feed rollers 336 and 337)(43), andthe flow returns to the step (41).

That is, when the K interruption flag is in the set state, the residualtoner collection operation is performed only to the K photosensitivedrum 345. Then, when the operation ends, the K interruption flag storedin the not-shown RAM of the image process unit 312 is reset, and theonce-interrupted printing job is restarted (i.e., restarting to supplythe recording member onto the transfer belt 333 by the sheet feedrollers 336 and 337), and the flow returns to the step (41).

On the other hand, when the YMC interruption flag is in the set state,the residual toner collection operation is performed to each of the Y, Mand C photosensitive drums 342, 343 and 344. Then, when the operationends, the YMC interruption flag stored in the not-shown RAM of the imageprocess unit 312 is reset, and the once-interrupted printing job isrestarted (i.e., restarting to supply the recording member onto thetransfer belt 333 by the sheet feed rollers 336 and 337).

Modification of Second Embodiment

In the above-described second embodiment, it is explained the case wherethe residual toner collection operation for K and the residual tonercollection operation for Y, M and C are independently performed inconsideration of the fact that the Y, M and C image formation operationsare not performed in a black and white mode. However, in a color imageformation apparatus which form (i.e., print) a black image by using theY, M and C photosensitive drums 342, 343 and 344, it may be structuredthat the residual toner collection operation for each of the Y, M and Cphotosensitive drums 342, 343 and 344 is performed when the imagedensity addition value of any of Y, M and C exceeds the predeterminedvalue.

Further, in the above embodiment, it is explained the case where theimage density is obtained by using the total number of light emission(i.e., video count) by the LED light emission elements of the imageformation apparatus which causes the LED light emission elements to emitthe light to form the latent image on the photosensitive member.However, in the apparatus which forms the latent image on thephotosensitive body by scanning the laser beam, it may be structuredthat the image density is obtained by using the total number of laserlighting signals (i.e., video count).

Further, it may be structured that the potential sensor is provided inthe vicinity of the photosensitive drum to measure the potential on thedrum, thereby obtaining the image density.

Thus, in the color image formation apparatus wherein the plural imagesupport bodies are disposed but the dedicated cleaning device for eachof these image support bodies is not disposed in the vicinity of thatbody, the density of the image to be formed on each image support bodyis detected, the printing job is temporarily interrupted according tothe density of the image formed on any image support body, and it isdetermined based on the image support body caused the interruptionwhether the residual toner collection operation for the specific imagesupport body is to be performed or the residual toner collectionoperation for the plural image support bodies is to be performed. Thus,it is possible to minimize deterioration of productivity and alsoprevent deterioration of the photosensitive drum caused by theunnecessary cleaning operation.

Further, it is possible to prevent that the toner is mixed with aferrite carrier acting as a dielectric brush in a charging unit, wherebyit is possible to maintain charging performance. Thus, it is possible toprevent image deterioration form a high-quality image.

Thus, in the color image formation apparatus wherein the plural imagesupport bodies are disposed and the dedicated cleaning device for eachof these image support bodies is not disposed, the printing job istemporarily interrupted according to the density of the image formed onany image support body, and it is determined based on the image supportbody caused the interruption whether the residual toner collectionoperation for the specific image support body is to be performed or theresidual toner collection operation for the plural image support bodiesis to be performed. Thus, it is possible to minimize deterioration ofproductivity and also prevent deterioration of the photosensitive drumcaused by the unnecessary cleaning operation.

Further, since a development agent collected into each of the respectivecolor development units by the residual toner collection operation canbe reused, it is possible to prevent that the development agent is usedwastefully, thereby satisfactorily saving the development agents.

In the above-described second embodiment, it is explained the examplethat the two kinds of flags (i.e., K interruption flag and YMCinterruption flag) are used. However, it is possible to use interruptionflags corresponding to respective image formation units. Namely, theresidual toner collection operation may be performed at independenttiming to each of the image formation units by using corresponding oneof K, Y, M and C interruption flags.

As explained above, according to the second embodiment, the pluraldetection means respectively detect the densities of the respectivecolor images formed on the different image support bodies by theprinting job, and the control means interrupts the printing job andcontrols the residual development agent collection operation into thedevelopment unit for any one of the image support bodies or the pluralimage support bodies in accordance with each addition value of eachimage density detected by the detection means. Therefore, the residualdevelopment agent collection operation for the unnecessary image supportbody is restricted, and the residual development agent collectionoperation for any one of the image support bodies or the plural imagesupport bodies is performed at the timing according to the image densityof each image support body, whereby it is possible to preventdeterioration of productivity and deterioration of the image supportbody.

Further, the collection means includes a first collection means (charger(magnetic field generation means, sleeve, low-resistance carrier)) whichonce captures the residual development agent on the image support body,changes its electrostatic characteristic of the captured toner, andbrings back the characteristic-changed development agent to the imagesupport body, and a second collection means (development unit) whichcollects the brought-back development agent. Therefore, it is possibleto prevent that the development agent is mixed with the low-resistanceferrite carrier acting as the dielectric brush in the charger, therebymaintaining charging performance.

Further, in the residual development agent collection operation to beperformed during interruption of the printing job, the control meanscontrols the charger such that, after the charger ejects theonce-captured development agent to the image support body, the chargeris controlled to further eject the development agent by driving thesleeve in the state that the magnetic field generation means is notdriven. Therefore, even in the residual development agent collectionoperation during the printing job interruption, it is possible tosufficiently eject the development agent which was used in thehigh-density image formation and captured into the charger in thedevelopment agent collection operation. Thus, it is possible to preventthat the development agent is mixed with the ferrite carrier acting asthe dielectric brush in the charger, thereby maintaining chargingperformance.

Further, in the control method for the image formation apparatus whichcomprises the respective-color image formation means for developing thelatent images formed on the different plural image support bodies forrespective color components in the printing job with use of respectivecolor development units and performing the multi-color image formationby transferring the developed latent images on the fed recording mediaand the respective-color collection means for collecting the residualdevelopment agents on the image support bodies into the respectivedevelopment units, the image density to be formed on each image supportbody is detected, the printing job is interrupted according to eachaddition value of the detected image density, and the residualdevelopment agent collection operation by the collection means isperformed to any one of the image support bodies or the plural imagesupport bodies. Therefore, the residual development agent collectionoperation to the unnecessary image support body is restricted, and theresidual development agent collection operation to any one of the imagesupport bodies or the plural image support bodies is performed at thetiming according to the image density of each image support body,whereby it is possible to minimize deterioration of productivity andprevent deterioration of the image support body.

Therefore, it is possible to restrict the residual development agentcollection operation to the unnecessary image support body, minimize thedeterioration of the productivity and prevent the deterioration of theimage support body.

Other Embodiments

As described above, needless to say, the present invention can becompleted in a case where a storage medium storing the program codes ofa software for realizing the functions of the above-describedembodiments is supplied to a system or an apparatus and a computer (CPUor MPU) in the system or apparatus reads and executes the program codesstored in the memory medium.

In such case the program codes themselves read from the storage mediumrealize the functions of the embodiments, and the storage medium storingsuch the program codes constitute the present invention.

The storage medium storing such the program codes can be, for example, afloppy disk, a hard disk, an optical disk, a magnetooptical disk, aCD-ROM, a CD-R, a magnetic tape, a non-volatile memory card, a ROM, anEEPROM or the like.

Needless to say, the present invention also includes, not only the casewhere the functions of the embodiments are realized by the execution ofthe read program codes by the computer, a case where an OS (operatingsystem) or the like functioning on the computer executes all the processor a part thereof according to the instructions of the program codes,thereby realizing the functions of the embodiments.

Needless to say, the present invention further includes a case whereinthe program codes read from the storage medium are once stored in amemory provided in a function expansion board inserted in the computeror a function expansion unit connected to the computer, and a CPU or thelike provided in the function expansion board or the function expansionunit executes all the process or a part thereof according to theinstructions of such program codes, thereby realizing the functions ofthe embodiments.

Further, the present invention can be applied to a system constructed byplural equipments or can be also applied to an apparatus comprising oneequipment. Also, needless to say, the present invention can be appliedto a case where a program is supplied to a system or an apparatus. Inthis case, by reading the storage medium which stores the programrepresented by the software to achieve the present invention into thesystem or the apparatus, the system or the apparatus can derive theeffects of the present invention.

Further, by downloading and reading the program represented by thesoftware to achieve the present invention from a data base on a network,the system or the apparatus can derive the effects of the presentinvention.

The present invention has been described in connection with the severalpreferred embodiments. The present invention is not limited only to theabove-described embodiments, but it is apparent that variousmodifications and applications are possible within the scope of theappended claims.

What is claimed is:
 1. An image formation apparatus comprising: imageformation means for performing image formation by developing a latentimage formed on an image support body with use of a development unit andby transferring the developed image onto a fed recording medium;collection means for collecting a residual development agent on theimage support body into the development unit; detection means fordetecting a density of the image to be formed on the image support body;and control means for causing said collection means to perform aresidual development agent collection operation, according to a numberof image formations of which image density detected by said detectionmeans exceeds a predetermined image density.
 2. An apparatus accordingto claim 1, wherein said collection means includes: first collectionmeans for once capturing the development agent residual on the imagesupport body, changing its electrostatic characteristic and againejecting the agent onto the image support body, and second collectionmeans for collecting the ejected development agent into a developmentunit of corresponding color.
 3. An apparatus according to claim 2,wherein said first collection means includes a charger for charging theimage support body.
 4. An apparatus according to claim 3, wherein saidcharger includes a magnetic field generator for generating magneticfield, a rotatable sleeve containing said magnetic field generator, anda low-resistance carrier disposed on the periphery of said sleeve andcapable of contacting with the image support body with predeterminedresistance.
 5. An apparatus according to claim 4, wherein said sleeve isrotatable in a direction opposite to a rotational direction of the imagesupport body.
 6. An apparatus according to claim 4, wherein said chargercharges the image support body by forming a dielectric brush with thelow-resistance carrier.
 7. An apparatus according to claim 4, wherein,in the residual development agent collection operation to be performedduring a printing job interruption, after said charger ejects theonce-captured development agent onto the image support body, saidcontrol means controls said charger to further eject the developmentagent by rotatably driving said sleeve in a state that said magneticfield generator is not driven.
 8. An apparatus according to claim 2,wherein said second collection means includes development units forrespective colors.
 9. An apparatus according to claim 1, wherein saiddetection means detects the density of the image to be formed on theimage support body, on the basis of a video count.
 10. An apparatusaccording to claim 9, wherein said image formation means forms thelatent image on the image support body by using plural light emissionelements arranged along a direction perpendicular to a feed direction ofthe recording medium, and said detection means uses as the video countthe total number of light emission of each light emission element. 11.An apparatus according to claim 9, wherein said image formation meansforms the latent image on the image support body by laser beam scanning,and said detection means uses as the video count the total number oflaser lighting signals.
 12. An apparatus according to claim 1, whereinsaid detection means measures a potential on the image support body, andthen said detection means detects the density of the image to be formedon the image support body, on the basis of the measured potential. 13.An apparatus according to claim 1, wherein said control means calculatesan addition value of image densities detected by said detection means,and controls a residual development agent collection operation time bysaid collection means in accordance with the calculated addition valueof the image densities.
 14. A control method for an image formationapparatus which comprises image formation means for performing imageformation by developing a latent image formed on an image support bodywith use of a development unit and by transferring the developed imageonto a fed recording medium, and collection means for collecting aresidual development agent on the image support body into thedevelopment unit, said method comprising: a detection step of detectinga density of the image to be formed on the image support body; and acollection step of causing the collection means to perform a residualdevelopment agent collection operation in accordance with a number ofimage formations of which image density detected in said detection stepexceeds a predetermined image density.
 15. An image formation apparatuscomprising: image formation means for each color for performingmulti-color image formation by developing a latent image ofcorresponding color formed on corresponding one of plural differentimage support bodies in a printing job with use of a development unitfor corresponding color and by transferring the developed image onto afed recording medium; collection means for each color for collecting aresidual development agent on the corresponding-color image support bodyinto the corresponding-color development unit; detection means for eachcolor for detecting a density of the image to be formed on thecorresponding-color image support body; and control means forcontrolling a residual development agent collection operation by saidcollection means to the specific one image formation means or to theplural image formation means, according to the image densities detectedby said detection means.
 16. An apparatus according to claim 15, whereinsaid collection means includes, first collection means for oncecapturing the development agent residual on the corresponding-colorimage support body, changing its electrostatic characteristic and againejecting the agent onto the corresponding-color image support body, andsecond collection means for collecting the ejected development agentinto the corresponding-color development unit.
 17. An apparatusaccording to claim 16, wherein said first collection means includes acharger for charging the corresponding-color image support body.
 18. Anapparatus according to claim 17, wherein said charger includes amagnetic field generator for generating magnetic field, a rotatablesleeve containing said magnetic field generator, and a low-resistancecarrier disposed on the periphery of said sleeve and capable ofcontacting with the corresponding-color image support body withpredetermined resistance.
 19. An apparatus according to claim 18,wherein said sleeve is rotatable in a direction opposite to a rotationaldirection of the corresponding-color image support body.
 20. Anapparatus according to claim 18, wherein said charger charges thecorresponding-color image support body by forming a dielectric brushwith the lowresistance carrier.
 21. An apparatus according to claim 18,wherein, in the residual development agent collection operation to beperformed during a printing job interruption, after said charger ejectsthe once-captured development agent onto the corresponding-color imagesupport body, said control means controls said charger to further ejectthe development agent by rotatably driving said sleeve in a state thatsaid magnetic field generator is not driven.
 22. An apparatus accordingto claim 16, wherein said second collection means includes thedevelopment units for respective colors.
 23. An apparatus according toclaim 15, wherein said corresponding-color detection means detects thedensity of the image to be formed on the corresponding-color imagesupport body, on the basis of a video count.
 24. An apparatus accordingto claim 23, wherein said image formation means forms the latent imageon the corresponding-color image support body by using plural lightemission elements arranged along a direction perpendicular to a feeddirection of the recording medium, and said corresponding-colordetection means uses as the video count the total number of lightemission of each light emission element.
 25. An apparatus according toclaim 23, wherein said image formation means forms the latent image onthe corresponding-color image support body by laser beam scanning, andsaid detection means uses as the video count the total number of laserlighting signals.
 26. An apparatus according to claim 15, wherein saidcorresponding-color detection means measures a potential on thecorresponding-color image support body, and then said detection meansdetects the density of the image to be formed on the image support body,on the basis of the measured potential.
 27. A control method for animage formation apparatus which comprises image formation means for eachcolor for performing multi-color image formation by developing a latentimage of corresponding color formed on corresponding one of pluraldifferent image support bodies in a printing job with use of adevelopment unit for corresponding color and by transferring thedeveloped image onto a fed recording medium, and collection means foreach color for collecting a residual development agent on thecorresponding-color image support body into the corresponding-colordevelopment unit, said method comprising: plural detection steps of eachdetecting a density of the image to be formed on the corresponding-colorimage support body; and collection step of causing the collection meansto perform a residual development agent collection operation to thespecific one image formation means or to the plural image formationmeans, according to the image densities detected in said detection step.28. An image formation apparatus comprising: image formation means forperforming image formation by developing a latent image formed on animage support body with use of a development unit and by transferringthe developed image onto a fed recording medium; collection means forcollecting a residual development agent on the image support body intothe development unit; detection means for detecting a density of theimage to be formed on the image support body; and control means forcausing said collection means to perform a residual development agentcollection operation, according to an image density detected by saiddetection means.
 29. An apparatus according to claim 28, wherein saidcollection means includes: first collection means for once capturing thedevelopment agent residual on the image support body, changing itselectrostatic characteristic and again ejecting the agent onto the imagesupport body, and second collection means for collecting the ejecteddevelopment agent into a development unit of corresponding color.
 30. Anapparatus according to claim 29, wherein said first collection meansincludes a charger for charging the image support body.
 31. An apparatusaccording to claim 30, wherein said charger includes a magnetic fieldgenerator for generating magnetic field, a rotatable sleeve containingsaid magnetic field generator, and a low-resistance carrier disposed onthe periphery of said sleeve and capable of contacting with the imagesupport body with predetermined resistance.
 32. An apparatus accordingto claim 31, wherein said sleeve is rotatable in a direction opposite toa rotational direction of the image support body.
 33. An apparatusaccording to claim 31, wherein said charger charges the image supportbody by forming a dielectric brush with the low-resistance carrier. 34.An apparatus according to claim 31, wherein, in the residual developmentagent collection operation to be performed during a printing jobinterruption, after said charger ejects the once-captured developmentagent onto the image support body, said control means controls saidcharger to further eject the development agent by rotatably driving saidsleeve in a state that said magnetic field generator is not driven. 35.An apparatus according to claim 29, wherein said second collection meansincludes development units for respective colors.
 36. An apparatusaccording to claim 28, wherein said detection means detects the densityof the image to be formed on the image support body, on the basis of avideo count.
 37. An apparatus according to claim 36, wherein said imageformation means forms the latent image on the image support body byusing plural light emission elements arranged along a directionperpendicular to a feed direction of the recording medium; and saiddetection means uses as the video count the total number of lightemission of each light emission element.
 38. An apparatus according toclaim 36, wherein said image formation means forms the latent image onthe image support body by laser beam scanning, and said detection meansuses as the video count the total number of laser lighting signals. 39.An apparatus according to claim 28, wherein said detection meansmeasures a potential on the image support body, and then said detectionmeans detects the density of the image to be formed on the image supportbody, on the basis of the measured potential.
 40. An apparatus accordingto claim 28, wherein said control means calculates an addition value ofimage densities detected by said detection means, and controls aresidual development agent collection operation time by said collectionmeans in accordance with the calculated addition value of the imagedensities.